Hint

Answer

Two cards: [ BRAIN ] and [ teaser ].

Let's examine this card by card...

[ BRAIN ] - this must have [ strain ] on the other side or else we would violate the rule, therefore we must turn this card over.

[ strain ] - this "can" have but doesn't require [ BRAIN ] on the other side. If it does, the rule is not violated. If it doesn't, the rule is still not violated. Therefore, we do not need to turn this card over.

[ MIND ] - will not violate the rule since it was stated that lowercase must be on the other side of this card. We do not need to turn this over.

[ teaser ] - this can violate the rule. First situation is not having [ BRAIN ] on the other side which does not violate the rule. Second situation is having [ BRAIN ] on the other side which DOES violate the rule. Therefore, we need to turn this one over too.Hide